Purification of perfluoroolefins and chloroperfluoroolefins



United States Patent Ofiice 3,300,525 Patented Jan. 24, 196i 3,300,538PURIFICATION OF PERFLUOROOLEFIN S AND CHLOROPERFLUOROOLEFINS YutakaKometani, Tatsno Sueyoshi, and Masayoshi Tatemoto, Osaka, Japan,assignors, by mesne assignments, to Thiokol Chemical Corporation,Trenton, N.J., a corporation of Delaware No Drawing. Filed Mar. 6, 1963,Ser. No. 263,121 7 Claims. (Cl. 260-6533) This invention relates to thepurification of fluoroolefin monomers and, more particularly, to aprocess for purifying monomeric perfiuoroolefins and fiuorochloroolefinsof the minute traces of olefinic impurities which tend to decrease thethermal stability of any polymer produced from such fluoroolefinmonomer. The invention also provides an improved process for purifyingtrifluorochloroethylene and tetrafiuoroethylene.

Among the many thermoplastic resins which are presently commerciallyavailable, the high molecular Weight polyfiuoroolefins are generallyrecognized by many polymer chemists as having outstanding physicalcharacteristics, although they are somewhat difficult to mold because oftheir high melting points. Frequently, molding polyfluoroolefins such aspolytrifiuorochloroethylene or polytetrafiuoroethylene requirestemperatures in excess of 300 C., at which temperatures there is often atendency for the polymer, upon cooling, to undergo a marked diminutionin such physical characteristics as its tensile strength. Thisphenomenon is especially noticeable when the polymer is subjected totemperatures in excess of 300 C. over prolonged periods of time.

During the exhaustive investigation which we and our colleagues at thelaboratories of Osaka Kinzoku Kogyo Co., Ltd, Osaka, Japan, haveconducted into various aspects of the production of monomericfluoroolefins and polyfluoroolefins, We have found that the inability ofpolyfluoroolefins to withstand heat aging at temperatures in excess of300 C. without undergoing any change in their physical properties is dueto the presence in the fluoroolefin monomer of minute traces of certainolefinic impurities which tend to decrease the thermal stability of anypolymer produced from such fluoroolefin monomer. By. gas chromatographicanalysis of various redistilled fluoroolefin monomers, we have been ableto identify these olefinic impurities (which are present in the monomerin concentrations as low as parts per million) as compounds in which theunsaturated carbon atoms contain at least one atom other than fluorineand carbon atoms. Each of these olefinic impurities is characterized byat least one hydrogen atom on one of the unsaturated carbon atoms and,moreover, contains no elements other than carbon, fluorine, chlorine andhydrogen. Apparently, the normal work-up of most fluoroolefin monomers,which are produced either by the pyrolysis of haloperfluoroalkanes or bythe dehydrohalogenation of hydrohaloperfluoroalkanes, is insufiicient toremove the last minute traces of these olefinic impurities.

We have now found that by treating the fluoroolefin monomer prior topolymerization with a sulfuric anhydride selected from the groupconsisting of sulfur trioxide, fuming sulfuric acid and the alkali metalpyrosulfates (or any such compound capable of generating sulfurtrioxide), it is possible to remove these olefinic impurities from themonomer and thereby improve the thermal stability of thepolyfluoroolefin produced by the radicalinduced polymerization of thetreated monomer.

Based on these discoveries, the invention provides an improved processfor purifying a fluoroolefin monomer of the minute traces of olefinicimpurities which tend to decrease the thermal stability of any polymerproduced from such fluoroolefin monomer, which comprises treat ing themonomer with a sulfuric anhydride selected fror the group consisting ofsulfur trioxide, fuming sulfuri acid, and the alkali metal pyrosulfates,to remove fror the monomer those olefinic impurities in which the unsaturated carbon atoms contain at least one atom othe than fluorine andcarbon atoms, thereby improving th thermal stability of thepolyfluoroolefin produced by th polymerization of such treated monomer.

Treatment of the fluoroolefin monomer with the sul furic anhydride maybe accomplished in the gaseous 0 liquid state, using either sulfurtrioxide (in any of it polymorphic forms), filming sulfuric acid, or thealkaI metal pyrosulfates, or any compound capable of genera! ing S0Although a wide range of process condition may be employed to purify thefluoroolefin monomer i: accordance with the invention, we have obtainedparticu larly satisfactory results by treating the fluoroolefin mono merprior to polymerization with from about 2 to abou 10 percent by weightof the sulfuric anhydride at a tem perature in the range from ambientroom temperatures t about 150 C. for a period of time in the range frorabout 10 seconds to about 30 minutes, and preferabl using about 5percent by weight of the sulfuric anhydrid at temperatures ranging fromroom temperature to abou C. for periods of time in the range from about3 seconds to about 10 minutes.

The following examples are illustrative of the improve ments which areobtained in the thermal stability of poly fluoroolefins when thefluoroolefin monomer is treatei prior to polymerization, in accordancewith the inventior with a sulfuric anhydride to remove the minute traceso olefinic impurities inherently present in such monomer:

EXAMPLE I 1,1,2-trifluoro-2-chloroethylene, which was prepared bdechlorinating 1,1,2 trifluoro 1,2,2 trichloroethane i methanol atreflux temperatures with zinc powder, an initially purified bydistilling the monomer and drying 1' over silica gel, was passed througha reaction tube (5 crr in diameter and 1.50 cm. in length) packed withRaschi rings (5 mm. in diameter and 10 mm. in length) togethe with 5percent by weight of sulfur trioxide, over a perioi of 5 minutes atambient room temperature and atmos pheric pressure. Upon recovery, thetreated monome was washed with water and then dried.

To illustrate the differences in the physical propertie ofpolytrifiuorochloroethylene prepared from treated an untreated monomer,two polymer samples (designated a A and B) were prepared under identicalreactio: conditions by separately polymerizing in vacuo 500 gram of SO-treated monomer (Sample A) and an equal weigh of untreated monomer(Sample B) in a 500-ml. autc clave, using from 0.05 to 0.3 gram oftrichloroacetyl per oxide at a temperature of -l5 C.:1 C. Each polymesample was then heat aged at 300 C. and atmospheri pressure and itsN.S.T. value (i.e. no strength tempera ture) measured before and afterthe heat treatment, usin the method described in the article by W. T.Millei Modern Plastics, p. 146, October 1954. In addition t themeasurement of the N.S.T. value in each test, the at sorption in theinfrared spectrum at 5.31 microns and a 4.43 microns were determined onfilms (0.1 mm. thick ness) of each polymer sample before and after thehes treatment. Since the absorption band at 5.31 micron characteristicof the radical, while the band at 4.43 microns is characteristi ofpolytrifiuorochloroethylene, the ratio of absorptions 2 5.31 micronswith respect to that at 4.43 microns is a1 indicia of the degree ofpyrolysis of the polymer. A1

ncrease in the N.S.T. value and a decrease in the ratio if absorption [D4.43] b th are indicative of deteriration of the polymer upon heattreatment. The results 1f these tests, which are summarized in Table I,clearly .emonstrate the improved thermal stability which is obained inpolytrifiuorochloroethylene (Sample A) preared from SO -treated monomerwhen compared to an :lentical polymer (Sample B) prepared from untreatedrifluorochloroethylene.

Table I HERMAL PROPERTIES OF POLYTRIFLUOROCHLOROETHYLENE PREPARED FROMTREATED AND UNTREATED MONOMER N .S/I.

Ratio of Sample Test N0. Treatment of absorption Physical monomer BeforeAfter spectra appearance heat heat Dam/D 4 treatment treatment Treatedwith S 310 317 0. 069 Unblistered.

d0 275 283 0.057 D0. Untreated. 314 277 0. 199 Do. 290 275 0. 121 D0.

268 243 0. 141 Blistered.

EXAMPLE II Tetrafluoroethylene, which was prepared by the pyrolsis ofchlorodifiuoromethane and initially purified by istillation, was passedthrough a reaction tube packed ith Raschig rings together With 5 percentby weight based on the weight of tetrafiuoroethylene) of sulfur ioxide,using the same reaction conditions described in Example I. The yield oftreated monomer was 98 perent of theory, based on the weight ofuntreated mono- 1er passed into the reaction tube.

Two polymer samples (again designated as A and B) were prepared underidentical reaction conditions y separately polymerizing in vacuo 125grams of SO eated tetrafluoroethylene (polymer Sample A) and an qualweight of untreated tetrafiuoroethylene (polymer ample B) in a 500-ml.stainless steel autoclave, using solution in 250-ml. of water of 0.003gram of potassium ersulfate and 0.001 gram of sodium bisulfate toinitiate olymerization. The polymerization reaction, which was arriedout at temperatures in the range from 0 C. to C. for 3 hours, yielded ineach instance 100 grams of olytetrafiuoroethylene.

Each polymer sample was then performed under the ressure of 300 kg./cm.and subjected to heat treatment t 380 C. for periods of time rangingfrom 0.5 hour 9 hours. Measurements were made on each sample t the endof the particular heat treatment of its tensile rength at the yieldingpoint and at the breaking point, well as is percent elongation at thebreaking point. he results of these physical tests are summarized inable H. Analysis of these results show the markedly nproved physicalproperties of polytetrafiuoroethylene Sample A) prepared from SO-treated monomer when ompared to polymer (Sample =B) prepared fromuneated tetrafluoroethylene.

Table II Although the foregoing examples have illustrated theeffectiveness with which the process of the invention may be used toproduce polytrifluorochloroethylene and polytetrafluoroethylene, equallysatisfactory results may also be obtained by similarly treating otherfiuoroolefin monomers to remove the minute traces of olefinic impuritieswhich tend to decrease the thermal stability of any polymer producedfrom such fiuoroolefin monomer.

We claim:

1. A process for purifying a fiuoroolefin monomer of the groupconsisting of perfluoi'oolefins and chloroperfiuor-oolefins of theminute traces of olefinic impurities which are characterized by at leastone hydrogen atom on one of the unsaturated carbon atoms and contain noelements other than carbon, fluorine, chlorine and hydrogen, and whichtend to decrease the thermal stability of any polymer produced from suchfiuoroolefin monomer, which comprises treating the monomer with fromabout 2 to about 10 percent by wegiht of a sulfuric anhydride selectedfrom the group consisting of sulfur trioxide, fuming sulfuric acid, andthe alkali metal pyrosulfates, at a temperature in the range fromambient room temperatures to about 150 C. for a period of time in therange from about 10 seconds to about 30 minutes to absorb from themonomer substantially all of such olefinic impurities withoutsubstantially absorbing the monomer in the sulfuric anhydride, therebyremoving substantially all olefinic impurities from the fiuoroolefinmonomer.

2. A process for purifying monomeric trifluorochl-oroethylene of theminute traces of olefinic impurities which are characterized by at leastone hydrogen atom on one of the unsaturated carbon atoms and contain noelements other than carbon, fluorine, chlorine and hydrogen, and whichtend to decrease the thermal stability of any polymer produced from suchtrifiuorochloroethylene monomer, which comprises treating the monomerwith from HYSICAL PROPERTIES OF POLYTETRAFLUOROETHYLENE PREPARED FROMTREATED AND UN- TREATED MONOMER Tensile strength at Tensile strengthElongation at Molecular weight Period of thermal yielding oint (kg/mm?)at break (kg/mm?) break (percent) (in ten thousands) treatment (hrs) A BA B A B A B about 2 to about percent by weight of a sulfuric anhydrideselected from the group consisting of sulfur trioxide, fuming sulfuricacid, and the alkali metal pyrosulfates, at a temperature in the rangefrom ambient room temperatures to about 150 C. for a period of time inthe range from about 10 seconds to about 30 minutes to absorb from themonomer substantially all of such olefinic impurities withoutsubstantially absorbing the monomer in the sulfuric anhydride, therebyremoving substantially all olefinic impurities from thetrifluorochloroethylene monomer.

3. A process according to claim 2, in which the monomerictrifluorochloroethylene is treated at a temperature in the range fromambient room temperatures to about 100 C. for a period of time in therange from about 30 seconds to about 10 minutes.

4. A process according to claim 3, in which the monomerictrifiuorochloroethylene is treated with about 5 percent by weight of thesulfuric anhydride.

5. A process for purifying monomeric tetrafluoroethylene of the minutetraces of olefinic impurities which are characterized by at least onehydrogen atom on one of the unsaturated carbon atoms and contain noelements other than carbon, fluorine, chlorine and hydrogen, and whichtend to decrease the thermal stability of any polymer produced from suchtertafluoroethylene monomer, which comprises treating the monomer withfrom about 2 to about 10 percent by weight of a sulfuric anhydrideselected from the group consisting of sulfur trioxide, fuming sulfuricacid, and the alkali metal pyrosulfates, at 4 temperature in the rangefrom ambient room tempera tures to about 150 C. for a period of time inthe rang from about 10 seconds to about 30 minutes to absorl from themonomer substantially all of such olefinic im purities withoutsubstantially absorbing the monomer i1 the sulfuric anhydride, therebyremoving substantially al olefinic impurities from thetetrafluoroethylene monomer 6. A process according to claim 2, in whichthe mono meric tetrafluoroethylene is treated at a temperature it therange from ambient room temperatures to about C. for a period of time inthe range from about 30 sec onds to about 10 minutes.

7. A process according to claim 6, in which the mono merictetrafluoroethylene is treated with about 5 percen by weight of thesulfuric anhydride.

References Cited by the Examiner UNITED STATES PATENTS 2,613,203 10/1952Myers 260-92. 2,613,232 10/1952 Janoski 260653.l 2,626,254 l/l953 Milleret al. 260-92.

OTHER REFERENCES Newer Methods of Preparative Organic Chemistry pages208-209, Interscience, NY. (1948).

JOSEPH L. SCHOFER, Primary Examiner.

I. F. MCNALLY, Assistant Examiner.

1. A PROCESS FOR PURIFYING A FLUOROOLEFIN MONOMER OF THE GROUPCONSISTING OF PERFLUOROOLEFINS AND CHLOROPERFLUOROOLEFINS OF THE MINUTETRACES OF OLEFINIC IMPURITIES WHICH ARE CHARACTERIZED BY AT LEAST ONEHYDROGEN ATOM ON ONE OF THE UNSATURATED CARBON ATOMS AND CONTAIN NOELEMENTS OTHER THAN CARBON, FLUORINE, CHLORINE AND HYDROGEN, AT WHICHTEND TO DECREASE THE THERMAL STABILITY OF ANY POLYMER PRODUCED FROM SUCHFLUOROOLEFIN MONOMER, WHICH COMPRISES TREATING THE MONOMER WITH FROMABOUT 2 TO ABOUT 10 PERCENT BY WEIGHT OF A SULFURIC ANHYDRIDE SELECTEDFROM THE GROUP CONSISTING OF SULFUR TRIOXIDE, FUMING SULFURIC ACID, ANDTHE ALKALI METAL PYROSULFATES, AT A TEMPERATURE IN THE RANGE FROMAMBIENT ROOM TEMPERATURES TO ABOUT 150*C. FOR A PERIOD OF TIME IN THERANGE FROM ABOUT 10 SECONDS TO ABOUT 30 MINUTES TO ABSORB FROM THEMONOMER SUBSTANTIALLY ALL OF SUCH OLEFINIC IMPURITIES WITHOUTSUBSTANTIALLY ABSORBING THE MONOMER IN THE SULFURIC ANHYDRIDE, THEREBYREMOVING SUBSTANTIALLY ALL OLEFINIC IMPURITIES FROM THE FLUOROOLEFINMONOMER.